489
FXUS63 KGRR 132314
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
714 PM EDT Sat Jul 13 2024

.KEY MESSAGES...

- Round of Thunderstorms Possible Overnight into Early Sunday

- Late Sunday Storms Possible Mainly Near/South of I-96

- Strong to Severe Storm Chances Monday

- Dry and Cooler End of the Week

&&

.DISCUSSION...
Issued at 315 PM EDT Sat Jul 13 2024

- Round of Thunderstorms Possible Overnight into Early Sunday

The first period of interest to watch will be late tonight into
Sunday morning. While severe weather is unlikely, thunderstorms
containing heavy downpours and some locally gusty winds are
possible. An isolated instance of damaging wind gusts around 60 mph
is possible mainly across the lakeshore counties.

An MCV is moving through north central WI this afternoon. Additional
scattered storms are occuring in MN. The primary feature to watch
into this evening and tonight will be where the MCV goes and what,
if any, additional storm development could occur along, ahead of,
and/or behind this feature. The 12z CAMs depict various outcomes,
with some suggesting little to no thunderstorm activity overnight
into Sunday morning, but the HRRR often has a superior handle on
convective mode and timing (and will also give a nod to the 00z
NAM 3km, which did well with location and extent of storms in MN
this morning). The MCV will head into an unstable environment in
eastern WI albeit with weak deep layer shear. In Michigan, our
deep layer shear looks poor even at 06z (less than 20 kts), but
does bump up to around 30 kts by 12z as we do see a modest 30kt
LLJ aim toward the region.

Around and after 06z, the HREF 4hr max dBZ with neighborhood
probability for over 40 dBZ (within 20 km of a point) tops out
around 50% across the CWA, indicating that a fair amount of the
membership has heavier storm activity over us but a fair amount
doesn`t. Again, am leaning on the better performing guidance for
this forecast period. Given the orientation of the MUCAPE gradient
overnight, could envision storm activity across northeastern and
east central Wisconsin moving E/SE over Lake Michigan and into west
central Lower Michigan, with additional backbuilding thunderstorms
possibly occurring as the LLJ feeds them. Will also be watching for
MCS development likely across MN overnight that could catch up to
existing activity in WI and press toward Lake Michigan, arriving
mid morning Sunday. If this scenario occurs, some heavy amounts
of rain with localized street/poor drainage flooding is not out of
the question. While some gusty winds could occur (primarily 40-50
mph), the 24 hr HREF LPMM tells the bigger story that the late
night and morning storms could easily lay down 1" or more in
spots. While location and geographic coverage is tough to pin
down, the favored area is the lakeshore but this does extend
toward US 131 also.

- Late Sunday Storms Possible Mainly Near/South of I-96

Better upper level synoptic support will be in place Sunday evening
and night as an upper jet with left exit region divergence moves in.
At 500 mb, another shortwave trough is shown in WI at 00z Monday and
a LLJ of 35-40 kts is forecast to move over northern IN. A
stationary boundary is slated to be draped over the state during
this time as well. Deep layer shear around 30 kts with SBCAPE of
2500 J/kg and PWATs of 1.8"-2.0" should be supportive for
thunderstorms especially toward late afternoon and evening, and
given the stronger wind fields aloft there is a better risk for
damaging wind gusts and hail especially from I-96 to the south. The
storms will be efficient rain producers, with yet another risk for
1" or more of rain in spots.

- Strong to Severe Storm Chances Monday

Another hot and humid day Monday with 850mb temps at 20C and
dewpoints surging into the mid 70s. A mid-level wave will push a
cold front through the region Sunday afternoon to Tuesday morning
while a warm front extends from MN/IA eastward towards northern IL.
To our west, thunderstorm activity will start off north of the warm
front and along/ahead of the cold front and will track eastwards
into WI and MI. Across our area substantial instability will build
to 3000+ J/kg with PWATs surging to 2+ inches ahead of the front
with bulk shear to 40 to 50 knots. Much of the setup will be
resultant on how prior convection evolves Saturday night and Sunday
night, therefore a fair amount of uncertainty exists on where the
exact corridor of thunderstorm activity will track. However,
generally the most likely corridor looks to be upstream convection
across MN/IA/WI propagating toward southwest Michigan during the
late afternoon to evening hours. Damaging wind gusts are the primary
threat.

- Dry and Cooler End of the Week

Upper troughing settles in over the region and cold air advection
brings in more comfortable conditions. Surface high pressure will
scour out any lingering moisture, so expect dewpoints back into the
50s. Plenty of sunshine into the end of the week apart from typical
diurnal cumulus. High temps in the 70s starting Wednesday.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 702 PM EDT Sat Jul 13 2024

A complicated aviation forecast is in store for the next 24 hours.
Currently widespread VFR conditions are ongoing with scattered
diurnal cumulus and high clouds across West Michigan. After 06z
tonight, the remnants of a convective system approach the area
bringing mid-level to high clouds across the area. Conceptually,
the decaying line of storms that has developed across WI and MN
will ride the instability gradient into SW Michigan. However
spread across solutions in coverage and strength of storms is
high. Will leave the current forecast as is with VCTS from 07z-14z
at MKG, AZO, BTL, and GRR. MVFR visbys are possible in any
storms. The HRRR guidance shows secondary development occuring
behind the line, however confidence in this scenario is to low to
include. Will reevaluate TAFs after the 00z suite of CAM guidance
comes in and amend if confidence grows in storm placement and
timing.

Mostly cloudy skies with clouds above 10kft expected tomorrow
after storms end with south-southwest winds of 10-15 knots
gusting to 25 knots. Weak signal for additional storms to develop
near AZO and BTL after 20z Sunday, however this is low confidence
as it depends on how morning storms go. Will leave this scenario
out of the TAFs but reevaluate and amend as needed.

&&

.MARINE...
Issued at 303 PM EDT Sat Jul 13 2024

Little wave action is expected for the remainder of today and
tonight given light winds. However, waves do look to pick up into
the 2-3 ft range Sunday as southwest winds increase to 10-20 kts.
At this point it does not look like winds will be strong enough or
waves high enough to reach SCA.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Air Quality Alert from midnight tonight to midnight EDT Sunday
     night for MIZ050-056-057-064-071.
MARINE...None.

&&

$$

DISCUSSION...Hoving
AVIATION...Thomas
MARINE...Hoving